In service, a tap changer is supplied with an input voltage and in response thereto produces an output voltage. The purpose of a tap changer is to produce an output voltage that is well regulated (i.e., substantially constant at some predetermined target level) despite fluctuations in the input voltage and load from their normal values. An AC voltage regulator for industrial use typically comprises a tap changer having a number of spaced-apart output terminals and performs its regulatory function by adjusting the tap position (in other words, tapping the output terminals at a selected position) so that, for a given input voltage, the output is taken from whichever tap yields an output voltage closest to the target level.
The number of taps provided depends on the environment in which the tap changer is designed to operate and the fineness or resolution with which it is necessary to control the output voltage. One type of tap changer in common use has the equivalent of 33 taps. These taps can be thought of as consisting of a centrally positioned neutral tap, 16 taps on one side of the neutral tap respectively corresponding to excursions of the input voltage of increasing magnitude in one direction from normal, and 16 taps on the opposite side of neutral respectively corresponding to excursions of the input voltage of increasing magnitude in the opposite direction from normal. In practice, such a tap changer has a neutral tap plus first through eighth additional taps and a reversing switch. The tap changer can be placed on the neutral tap to yield an output voltage equal to the input voltage. With the reversing switch in the "raise" position, the tap changer can be placed on the neutral and first taps for a one-raise, entirely on the first tap for a two-raise, on the first and second taps for a three-raise, entirely on the second tap for a four-raise, and so on until the tap changer is entirely on the eighth tap for a sixteen-raise. With the reversing switch in the "lower" position to reverse the current through the coil, the tap changer can be moved in the same way over the same taps to obtain any lower position ranging from a one-lower to a sixteen-lower.
The dynamic range at the input side is typically the normal input voltage plus or minus 10%. When the input voltage is at its normal value, the voltage regulator tap position is normally in neutral and the output voltage of the voltage regulator is equal to the input voltage.
Operators of large industrial electrical installations employing voltage regulators with tap changers need information about tap positions because of its bearing on economy of operation, maintenance, safety, and system performance analysis. Consider the matter of economy of operation. Sometimes, because of poor performance of a voltage regulator, power is supplied at a voltage which, although not so high as to damage the electrical components that receive power from the tap changer, is higher than the voltage required. In such a case, more power is delivered than is necessary, and the excess power is wasted. In a large industrial application, the waste can be quite substantial.
From the standpoint of maintenance and safety, in certain circumstances it is necessary to move the tap changer quickly and reliably to its neutral position. It is essential that the tap changer position be in neutral whenever the voltage regulator is placed in or removed from service. Information about current tap position is necessary to accomplish this. From the standpoint of system performance analysis, a record of the successive active tap positions of a tap changer is a useful measure of the range and frequency of input voltage excursions and load changes, which are related respectively to the performance of the power supply to the tap changer and to the performance of the system to which the tap changer supplies power.
Various kinds of apparatus have been developed in the past for determining the tap position of a tap changer. These prior developments have culminated in the standard electromechanical tap position indicator, which is physically attached to the tap changer mechanism, a mechanical device that changes the tap position by physically moving from tap to tap. The attached tap position indicator moves with the tap changer mechanism and displays the tap position on a dial or in some other conventional manner.
The standard, conventional electromechanical meter has a number of drawbacks. For one, it has costly moving parts that wear out and is inherently less reliable and more expensive than desirable. Moreover, it produces only a local meter indication, which can be read by an operator only by going to the site of the meter. Furthermore, if meter readings are converted into a signal that can be transmitted to a remote location for reading or to a centrally located computer for processing, such conversion must be performed reliably and cost effectively.
Other prior art relating to the monitoring or determination of the tap position of a tap changer is found in U.S. Pat. Nos. 4,419,619, 4,612,617 and 5,119,012. The devices shown in these patents all have various drawbacks, including relative complexity and a failure to provide certain information or a failure to provide information in a form desired by operators of large industrial installations incorporating voltage regulators.
In view of the foregoing, it would be desirable to provide a remedy for the problems of the prior art outlined above. In particular, it would be desirable to provide improved monitoring apparatus and methods for use with a tap changer that reliably and inexpensively keep track of the tap position as it changes. Furthermore, it would be desirable to provide monitoring apparatus that provides information on tap position in a form that is convenient and easily accessible either at the tap changer or at a remote location to elevate standards of economy, maintenance, safety, and system performance analysis.